One type of state-of-the art flow control valves, in particular air control valves for ventilation ducts, typically comprises a valve plate which is arranged in the flow duct in such a way that the plate is rotatable around an axis which extends transversely to the fluid flow direction through the duct and valve. For applications requiring accurate flow control, so-called iris valves are sometimes applied, which are typically designed on the basis of a plurality of mutually displaceable plates, whereby, depending on the position of the plates, a clearance of a variable size is provided for the fluid flow. In respect of relatively large ducts, louvre valves are often used, which include a plurality of rotatable plates, which are arranged and designed to entirely close the duct.
Most of such valves do, however, result in a relatively large flow resistance, i.e. energy loss, as well as unsatisfactory generation of aerodynamic noise as a consequence of sharp edges provided by the valve plates.
Venturi valves are used in demanding applications, such as in laboratory exhaust systems. Such valves include a venturi nozzle arrangement with a movable cone, which is displaceable in the nozzle, whereby the flow rate of the fluid through the duct is controllable. Venturi valves are, however, relatively expensive, and their installation is complicated.
Pneumatic membrane valves or sliding valves are also known. Membrane valves operate by forcing a flexible membrane surrounding the fluid flow to open or close by application of air pressure. Such valves suffer from the drawback that they require a complex and expensive pneumatic installation for controlling the membrane. Sliding valves include a translationally movable plate, which is arranged to open or close a passage having the size of the plate, e.g. by means of a pneumatic cylinder. Its installation is, however, complex and expensive.
Flow meters are sometimes installed in connection with flow control valves. Though flow meters are often required, they add to the overall costs of the flow control equipment, and provide additional pressure loss.